Low molecular weight glycol ethers and their acetate derivatives are widely used as solvents in paints and coatings. In 1984, the EPA issued guidelines to reduce exposure to 2-methoxyethanol, 2-ethoxyethanol and their acetates since studies indicated these chemicals were carcinogens.
Ethylene glycol monobutyl ether plays an important role in waterborne coatings, accounting for about 80% of glycol ether content, but has not been implicated in any studies as a tetrogen. Many solvents have been reformulated in recent years to use heavier propylene glycol and ethylene glycol monobutyl ethers rather than lighter ethylene glycol monomethyl and ethyl ethers (see for example European Patent Application 288-856A).
Methods of preparing ethylene and propylene glycol mono-n-butyl ethers are known in the art. For example, one method for preparation is by the reaction of ethylene oxide or propylene oxide and n-butanol.
Another method of synthesizing low molecular weight ethers is disclosed in U. S. Pat. No. 4,714,787 which discloses a process for selectively reacting one or more linear monoolefins with a primary or secondary lower molecular weight alcohol to form the corresponding ethers. The active acidic catalyst component is a sulfonate ion-exchange resin or a crystalline silicate having a pore size greater than 5 A.U. In the preferred embodiment, methanol and propylene are reacted to selectively form methyl isopropyl ether and the preferred crystalline silicate includes a crystalline zeolite having a silica to alumina mole ratio greater than about 12. Here the main products are methyl isopropyl ether and dimethyl ether.
In U. S. Pat. No. 4,675,082 there is disclosed a method for preparation of 1-t-butoxy-2-propanol from the etherification of propylene glycol with isobutylene in the presence of a solid resin etherification catalyst, typically an acidic ion exchange resin such as AMBERLYST.RTM.-15, consisting of sulfonated polystyrene matrix having up to about 25% of copolymerized divinylbenzene bearing functional sulfonic acid groups contained therein. This process requires a two tower separation.
Polyethylene glycol and propylene are reacted to form polyethylene glycol dipropyl ethers using a strongly acidic cation resin as a catalyst. This process provides good yields of polyethylene glycol dipropyl ethers by reducing side reactions such as polymerization and decomposition. See reference (J58052321-A or J8 8037819-B) in Derwent Japanese Patents Abstract, Polymer+General Chemistry, Vol. 88, No. 30. In a Japanese reference to the same company, Nippon Shokubai Kagaku, polyethylene glycol is reacted with isobutylene to produce polyethylene glycol dibutyl ether (J58049725-A).
The preparation of propylene glycol tert-butyl ether from propylene glycol and isobutylene is disclosed in Jpn. Kokai Tokkyo Koho JP 63,250,336 to Fujiwara, Hiroshi et al. (Maruzen). A strongly acidic cation-exchanger resin is employed along with tert-butanol (Me.sub.3 COH) which serves to inhibit the formation of by-product diisobutylene. The reaction product must be distilled to separate propylene glycol tert-butyl ether from propylene glycol di-tert butyl ether.
Methods of using clays as catalysts for certain reactions are known in the art. In Chem. Systems, Topical Reports, Vol. II, 1986 Program (May 1987), Section 3.00, there is an article which introduces the subject of pillared clays. There it is disclosed that several processes can employ these pillared clay catalysts and that propylene glycol ethers are of particular interest, since, as mentioned, corresponding ethylene glycol ethers are believed to be toxic. Further, it is stated that alkylene glycol ethers are conventionally prepared by the interaction of an alcohol with an epoxide. Generally a catalyst such as sodium hydroxide or an alkali metal alcoholate is used, together with a tenfold excess of alcohol.
There is art available which has focused on how various factors affect clay catalysts. In an article titled "Pillared Clays As Catalysts" in Catal. Rev.-Sci. Eng., 30(3), 457-499 (1988) there is a discussion of factors affecting the thermal stability of pillared clays and how the stability can be improved in the range from about 480.degree. C. to about 800.degree. C. The same article also discusses the acidity of pillared clays and ways in which different treatments affect the Lewis or Bronsted sites to a varying extent.
Stabilized pillared interlayered clays are used in the invention of EP 0083 970 to carry out processes capable of catalysis by protons. The invention included methods for preparing alcohols from olefins or olefin oxides and water, or ethers from primary or secondary aliphatic alcohols, a polyol or an olefin oxide.
An interesting comparison of montmorillonite-derived catalysts with ion-exchange resins as it relates to one particular reaction is found in an article titled "Methyl-t-Butyl Ether (MTBE) Production: A Comparison Of Montmorillonite Derived Catalysts With An Ion-Exchange Resin", Adams, et al. Clays and Clay Minerals, Vol. 34., No. 5, 597-603, 1986. Here it is concluded that Al.sup.+3 -clay has greater catalytic activity than acid treated-clay (K10) which has greater activity than ACH-treated clay which is approximately the same as pillared clay. Further, the activity of Al.sup.+3 montmorillonite at 60.degree. C. is about 50% of that exhibited by an acidic ion-exchange resin similar to that used industrially.
Research performed by Adams, et al., at the University of Wales involved the acid-catalyzed reaction between methanol and isobutene to give methyl-t-butyl ether. These researchers found that where cation-exchanged smectite was used as the catalyst, the clays which had been exchanged with monovalent divalent cations gave low yields in comparison with clays exchanged with trivalent ions. See "Synthesis Of Methyl-t-Butyl Ether From Methanol And Isobutene Using A Clay Catalyst", Adams, et al., Clay and Clay Minerals, Vol. 30, No. 2, 129-134, 1982.
It is always advantageous to prepare chemicals for which there is a demand from inexpensive feedstocks or feedstocks that are readily available, or a by-product of some chemical process. It would be a distinct advance in the art if a process were available which allowed for the continuous production of low molecular weight glycol monoalkyl ethers, particularly glycol mono-t-butyl and mono-isopropyl ethers in high selectivity and conversion from readily available, and inexpensive, oxirane/olefin feedstock combinations. Furthermore, it would be advantageous if this reaction was one-step, did not require any second distillation and the catalyst was physically and chemically stable to the reactant/product media over a broad range of temperatures, particularly above 100.degree. C., so that there were minimal problems with catalyst decomposition and loss activity.
It has been surprisingly discovered that several different classes of acidic catalysts can be used to synthesize ethylene and propylene glycol ethers from the corresponding olefins and oxiranes. It is particularly an object of the present invention to recover mono-t-butyl ethers in a high state of purity with minimal amounts of by-products. Other objects will become apparent to those skilled in the art from the following description.